One of the very important changes taking place as digital technology is extended from digital switching systems to digital access lines serving fully digital user stations, is the replacement of traditional analog station signaling, involving, for example, the transmission and detection of analog signals representing on-hook and off-hook states, by digital signaling procedures where complete digital messages are exchanged to control calls.
Initially, such digital message signaling was of a type referred to as stimulus signaling where each message is either generated as a result of a single event at a user terminal or contains a basic instruction from the switching system to be executed by a user terminal. Examples of such stimulus messages include a message generated by a user terminal defining the activation of a particular button, and a message from the switch, instructing a user terminal to flash a given lamp. With stimulus signaling, all the intelligence giving meaning to the various user terminal events resides in the switch. The switch processes stimulus messages defining button activations by reading stored data defining, for example, that the activation of button number 8 at a given user terminal represents a request for a conference call or that button number 12 is a call appearance at the given terminal and its activation represents a preselection of that call appearance by a user prior to initiating a call. The activation of button number 8 and button number 12 at other user terminals may have completely different meanings. Placing all the intelligence concerning terminal configuration in the switch is desirable since all system users can be given identical terminals and the terminals can be configured in many ways to accommodate different user requirements.
Because of the incorporation of microprocessors in more sophisticated user terminal equipment, a second type of message signaling, referred to as functional signaling, has evolved which involves a degree of intelligent processing in its generation or analysis. With functional signaling, the switch and the user terminal communicate with each other as peers. The user terminal intelligence moves through a number of call states as the call progresses from origination to disconnection. The messages generated in response to events at the user terminal are context-dependent, i.e., the message generated in response to a given event depends on the user terminal call state. With functional signaling, the user terminal knows the meaning of user events such as the activation of call appearance and feature buttons, and sends appropriate functional messages to the switch in response to such events. For example, when a particular call appearance is activated at a user terminal that is off-hook and a destination directory number is subsequently entered, the user terminal may transmit a message requesting that the switch set up a call from the particular call appearance to the destination directory number. When a conference button is activated, the user terminal transmits a message requesting a conference call. The increased user terminal intelligence and the use of functional messages advantageously reduce the magnitude of call processing activity required of the switching system and facilitate the evolution of a wide variety of complex features. However, functional signaling systems do not have the terminal configuration flexibility afforded when all terminal configuration data resides in the switch as with stimulus signaling systems. With functional signaling, each user terminal must be individually programmed such that the terminal knows the meaning of the particular configuration of buttons at that terminal.
In view of the foregoing, a recognized need in the art exists for a message signaling arrangement that allows intelligent peer-level communication between a switching system and user terminals without having to store data defining terminal configuration in the individual terminals.